David Preswick Barr was born in Ithaca, New York on August 23, 1889, the oldest son of Fred C. and Priscilla Preswick Barr. Fred Barr owned a local hardware store and his son seems to have been raised in relative comfort. After attending Ithaca's public schools, Barr entered Cornell University from which he graduated with an A.B. in 1911. He then entered the Ithaca branch of Cornell University Medical College (CUMC), transferring to the New York City campus the next year.

He received his M.D. in 1914 and for the next two years was a house officer in the Cornell or 2nd Medical Division of Bellevue Hospital. He was then appointed both a research fellow at the Russell Sage Institute for Pathology at Bellevue Hospital and instructor in medicine at CUMC. At Russell Sage he worked with Graham Lusk and Eugene DuBois, among others, in exploring metabolic and physiological changes occurring in disease states.

With a two year hiatus for active military medical service during WWI, Barr remained at Russell Sage and CUMC until 1924. He then became Professor and Chairman of the Department of Medicine at Washington University Medical School in St. Louis, Missouri. He was at the time the youngest chairman ever to have been appointed in a U.S. medical school. During his tenure at Washington University he helped lead the medical school to national prominence as well as edit the massive, three-volume "Modern Medical Therapy in General Practice."

Barr returned to The New York Hospital-Cornell Medical Center in 1941 as Professor and Chairman of the Department of Medicine. He became an advocate of outpatient care in clinics and of health insurance. His attempts to inaugurate a prepaid system of comprehensive health care at New York Hospital were at first stymied by internal opposition. By the end of the 1940s, however, with help from the Commonwealth Fund, he had established such an outpatient clinic at the hospital.

"Retirement" in 1957 was not the end of Barr's extraordinary career. He was President of the Health Insurance Plan of Greater New York, 1957-1962, and Visiting Professor at the National Defense Medical Center in Taiwan, 1963-1966.

By all accounts, Barr was genuinely beloved by his colleagues and students at the New York Hospital -Cornell Medical Center. His Friday afternoon house staff parties, which he dubbed "Liver Conferences," were legendary. He married twice, first to Mary Washington Walker, who died in 1954 and then, in 1956, to Fredrica (Jean) Steadman, a fellow employee at the Center. Barr was an enthusiastic gardener, an excellent amateur painter (an avocation he took up in his 70s) as well as a fine piano player and singer.